Tetrahydrothiophene-based GABA aminotransferase inactivators and analogs thereof for treating neurological disorders

ABSTRACT

Tetrahydrothiophene and related heterocyclic analogs and related methods for GABA aminotransferase inactivation.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 15/712,793, filed on Sep. 22, 2017, published as US 2019/0051001, onFeb. 22, 2018, which application is divisional application of U.S.application Ser. No. 15/065,487, filed on Mar. 9, 2016, published asU.S. 2016/0264544, published on Sep. 15, 2016, and issued as U.S. Pat.No. 9,856,231, on Jan. 2, 2018, which application claims the benefit ofpriority to U.S. Provisional Application No. 62/130,219 filed on Mar. 9,2015, the entirety of which applications are incorporated herein byreference in their entireties.

This invention was made with government support under grant number R01DA030604 awarded by the National Institutes of Health. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Epilepsy is a family of chronic neurological disorders characterized byrecurring convulsive seizures, which result from abnormal, excessiveneuronal activity in the central nervous system. It is estimated thatabout 65 million people worldwide have epilepsy. Epilepsy can arise froman imbalance in two major neurotransmitters that regulate brain neuronalactivity, L-glutamate, an excitatory neurotransmitter, andγ-aminobutyric acid (GABA), an inhibitory neurotransmitter.

GABA is produced in GABAergic neurons from L-glutamate by the enzymeglutamic acid decarboxylase (GAD) (FIG. 1). GABA is then released intothe synapse and transported to glial cells. The enzyme GABAaminotransferase (GABA-AT) in glial cells degrades GABA to succinicsemialdehyde (SSA), which is further oxidized to succinate and entersthe Krebs cycle. GABA-AT also converts α-ketoglutarate from the Krebscycle to L-glutamate. Because there is no GAD in glial cells, this newlyformed L-glutamate is not converted to GABA. It is instead converted toL-glutamine, which is then released from glial cells into the synapseand transported back to GABAergic neurons to complete the metaboliccycle of L-glutamate.

Low levels of GABA are linked to not only epilepsy, but also many otherneurological disorders including Parkinson's disease, Alzheimer'sdisease, Huntington's chorea, and cocaine addiction. Raising GABA levelshas proven effective in stopping recurring convulsive seizures in thetreatment of epilepsy. However, GABA does not cross the blood-brainbarrier (BBB); therefore, an increase in brain levels of GABA cannot beachieved by intravenous administration. Other possible routes toincreased brain levels of GABA include enhancing the activity of GAD,the enzyme that makes GABA, inhibiting the activity of GABA-AT, theenzyme that degrades GABA, and inhibiting the reuptake of GABA byblocking the action of the GABA transporters.

One approach relates to the design of mechanism-based inactivators ofGABA-AT; in particular, the design of unreactive compounds that requireGABA-AT catalysis to convert them into a species that inactivates theenzyme. Because these molecules are not initially reactive, but requirethe catalytic activity of GABA-AT to become activated and form covalentbonds, indiscriminate reactions with off-target proteins, leading toundesired side effects, should be greatly reduced. Even at lowerdosages, these inactivators can achieve the desired pharmacologiceffects with enhanced potency and selectivity than conventionalinhibitors.

Currently, the only FDA-approved inactivator of GABA-AT is the drugvigabatrin (2) (Scheme 1), which was first developed by Lippert et al.,and is used for the treatment of epilepsy. However, a large dose ofvigabatrin (˜1-3 g) needs to be taken daily, and there are many seriousside effects that arise from its usage, including psychosis andpermanent vision loss resulting from the damage of the retinal nervefiber layer. As a result, the search for an alternative to vigabatrin inthe treatment of epilepsy has been an ongoing concern in the art.

It has been determined that vigabatrin inactivates GABA-AT via twopathways: a Michael addition mechanism and an enamine mechanism, asshown in Scheme 1. In the Michael addition mechanism, the resultingSchiff base (4) from the reaction of vigabatrin and the lysine-bound PLP(1) on GABA-AT is subjected to γ-proton removal and tautomerization thatleads to ketimine 5. An active-site nucleophile then reacts with Michaelacceptor 5 to form 6, which is in equilibrium with 7. In the enaminemechanism, the Schiff base (4) is subjected to γ-proton removal andtautomerization through the vinyl bond, which leads to the release ofenamine 10. Subsequent nucleophilic addition of 10 to the lysine-boundPLP on GABA-AT gives rise to 11.

The Michael addition mechanism and the enamine mechanism happenconcurrently in a 70/30 ratio, respectively. It was discovered thatketimine 5 in the Michael addition mechanism, and enamine 10 in theenamine mechanism, underwent partial hydrolysis to form theα,β-unsaturated ketone (8) and the saturated ketone (12), respectively.While 8 is a reactive electrophile, possibly responsible for some sideeffects, 12 is not a reactive metabolite. From these findings, furtherstudy has been directed to vigabatrin analogs that either follow theenamine mechanism exclusively to avoid the formation of 8 or speed upthe Michael addition pathway so that 5 would have much lower probabilityto undergo hydrolysis.

An energy minimized molecular model of vigabatrin bound to PLP inGABA-AT revealed that after tautomerization, the vinyl bond in 5 needsto rotate toward Lys-329 for the Michael addition to occur. Therefore,conformationally-restricted analogs such as 13 and 14 (FIG. 2) wouldprevent the rotation of the vinyl bond, thereby blocking the Michaeladdition mechanism. Experiments showed that 13 inactivated GABA-ATfollowing the enamine mechanism exclusively. However, its potencyremained low. In the alternative approach, conformationally-restrictedanalogs 15 and 16 have the vinyl bond readily pointed toward Lys-329 forrapid Michael addition to occur, thereby minimizing the hydrolysis ofthe ketimine intermediate. Experiments showed that 16 was 186 times moreefficient in inactivating GABA-AT than vigabatrin. Furthermore, unlikevigabatrin, 16 did not inactivate or inhibit off-target enzymes, such asalanine aminotransferase and aspartate aminotransferase, and thereforeis less likely to produce side effects. Indeed, 16 was tested in amultiple-hit rat model of infantile spasms, and the results showed that16 suppressed spasms at doses of 0.1-1 mg/kg/day, which were >100-foldlower than those for vigabatrin. The spasms suppression by 16 stayedeffective longer (3 days vs. 1 day for vigabatrin), and 16 also had amuch larger margin of safety than vigabatrin.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide compounds, compositions and related methods of use for theselective inactivation of GABA-AT, thereby overcoming variousdeficiencies and shortcomings of the prior art including those outlinedabove. It would be understood by those skilled in the art that one ormore aspects of this invention can meet certain objectives, while one ormore other aspects can meet certain other objectives. Each objective maynot apply equally, in all its respects, to every aspect of thisinvention. As such, the following objects can be viewed in thealternative with respect to any one aspect of this invention.

It is an object of the present invention to provide one or more smallmolecule compounds exhibiting GABA aminotranferase inactivation.

It can be another object of the present invention to provide one or moresuch compounds for in vitro use and study under conditions indicative ofone or more mammalian disease states.

Alternatively, it can also be an object of the present invention toprovide one or more such compounds enabling in vivo treatment of suchdisease states.

It can also be an object of the present invention to provide one or moresuch compounds with structural features facilitating interaction withand inactivation of GABA-AT.

It can also be an object of the present invention, alone or inconjunction with one or more of the foregoing objects, to provide acompound or composition for GABA-AT inhibition or inactivation,modulation of GABA-AT activity and/or treatment of epilepsy and variousother neurological disorders and indications.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and the followingdescriptions of certain embodiments of such compounds, compositionsand/or methods and will be readily apparent to those skilled in the arthaving knowledge of the synthetic techniques described herein. Suchobjectives, features, benefits and advantages will be apparent from theabove as taken into conjunction with the accompanying examples, data,figures and references incorporated herein, together with all reasonableinferences to be drawn therefrom.

In part, the present invention can be directed to a compound of aformula

wherein X can be selected from S, SO₂, CH₂, O, and NR, where R can beselected from H and C₁-about C₆ alkyl moieties, including withoutlimitation such straight-chain and branched alkyl moieties, or a saltthereof. In certain embodiments, X can be S. Without limitation incertain such embodiments, the amino and carboxy substituents can have acis or trans stereochemical relationship.

In part, the present invention can also be directed to a compound of aformula

wherein in the amino and carboxy substituents can have either a cis or atrans relationship, or a salt thereof. In certain embodiments, such acompound can be of a formula

wherein the amino and carboxy substituents can have a cis stereochemicalrelationship, or a salt thereof.

Regardless, compounds useful in conjunction with this invention arewithout stereochemical or configurational limitation. As illustrated anddiscussed below, such compounds and/or their intermediates are availableas single enantiomers, racemic mixtures from which isomers can beresolved, or diastereomers from which the corresponding enantiomers canbe separated. Accordingly, any stereocenter can be (S) or (R) withrespect to any other stereocenter(s). The amino and carboxy substituentscan have either a cis or trans stereochemical relationship. As anotherseparate consideration, various compounds can be present as an acidsalt, either partially or fully protonated. In certain such embodiments,with respect to an ammonio substituent, the counter ion can be aconjugate base of a protic acid. In certain such or other embodiments,with respect to a carboxylate substituent, the counter ion can be analkaline, alkaline-earth or ammonium cation. As described below, such acompound can be an amino acid hydrochloride. Further, it will beunderstood by those skilled in the art that any one or more thecompounds of this invention can be provided as part of a pharmaceuticalcomposition comprising a pharmaceutically-acceptable carrier componentfor use in conjunction with a treatment method or medicament.

In part, the present invention can be directed to a method for thetreatment of a neurological disorder in a subject in need thereof. Sucha method can comprise administering to such a subject a compound of aformula

wherein X can be selected from S, SO₂, CH₂, O, and NR, where R can beselected from H and C₁-about C₆ alkyl moieties, including withoutlimitation such straight-chain and branched alkyl moieties, or a saltthereof. In certain embodiments, X can be S. Without limitation, incertain such embodiments, the amino and carboxy substituents can have acis or trans stereochemical relationship.

In part, the present invention can be directed to a method for thetreatment of a neurological disorder in a subject in need thereof. Sucha method can comprise administering to such a subject a compound of aformula

wherein the amino and carboxy substituents can have either a cis or atrans stereochemical relationship, or a salt thereof. Withoutlimitation, in certain such embodiments, the amino and carboxysubstituents can have a cis stereochemical relationship.

In part, the present invention can also be directed to a method ofreducing or modulating activity of a GABA aminotransferase. Such amethod can comprise providing a compound of the sort discussed above ordescribed elsewhere herein; and contacting such a compound with a mediumcomprising GABA aminotransferase with an amount of such a compoundeffective to reduce or modulate GABA aminotransferase activity. Such amethod can thereby reduce or modulate succinic semialdehyde and/orglutamate production in such a medium. In certain embodiments, such acompound can be provided as part of a pharmaceutical composition.Regardless, such contact can be in vitro or in vivo.

More generally, the present invention can also be directed to a methodof reducing or modulating activity of GABA aminotransferase expressed bya glial cell. Such a method can comprise providing a compound of thesort discussed above or described elsewhere herein; and contacting sucha compound with a cellular medium comprising glial cells with an amountof such a compound effective to reduce or modulate GABA aminotransferaseactivity. Such a method can thereby reduce or modulate succinicsemialdehyde and/or glutamate production in such a cellular medium. Incertain embodiments, such a compound can be provided as part of apharmaceutical composition. Regardless, such contact can be in vitro orin vivo.

More generally, the present invention can also be directed to a methodinhibiting or inactivating a GABA aminotransferase. Such a method cancomprise providing a compound of the sort discussed above or describedbelow, whether or not part of a pharmaceutical composition, andadministering an effective amount of such a compound for contact with aGABA aminotransferase. Such contact can be, as would be understood inthe art, for experimental and/or research purposes or as may be designedto simulate one or more in vivo or physiological conditions. Suchcompounds can include but are not limited to those illustrated by thefollowing examples, referenced figures, incorporated references and/oraccompanying synthetic schemes. In certain such embodiments, such acompound and/or combination thereof can be present in an amount at leastpartially sufficient to inactivate GABA-AT, or inhibit or modulate GABAdegradation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Metabolic cycle of L-glutamate.

FIG. 2. Vigabatrin analogs (prior art) that follow one GABA-ATinactivation mechanism exclusively.

FIG. 3. Tetrahydrothiophene-based GABA analogs, in accordance withcertain embodiments of this invention.

FIGS. 4A-C. The PLP-dihydrothiophene complex, shown in ball-and-stickform, is the final adduct, which contains a nonplanar dihydrothiophenering. (A) The electron density of the simulated-annealing omit map(F_(O)-F_(C)) is shown as a gray mesh at 3.5σ around thePLP-dihydrothiophone adduct. (B, C) Two different orientations of thesame image, displaying (B) the refined atom positions and (C) thebuckled ring plane. The electron density of the simulated omit map(F_(O)-F_(C)) is shown in a mesh at 4.16σ around atoms in thedihydrothiophene ring (at a radius of 0.6 Å).

FIGS. 5A-B. (A) Interactions between the PLP-dihydrothiophene adduct andnearby residues. (B) Directionality of the intermolecular weak nonbondedS . . . O interaction in theoretical studies, representing anπ_(O)→σ_(s)* orbital interaction.

FIG. 6. Radioactive-labeling experiment for the inactivation of GABA-ATby 17: [7-³H]PLP-GABA-AT was prepared from apoGABA-AT and [7-³H]PLP theninactivated by 17, followed by denaturation and submission to HPLC.Fractions were collected each minute and counted for radioactivity. Asolution of 1 mM PMP and 1 mM PLP was treated identically as a control.

FIG. 7. Overlay of in silico model of compound PLP-39 adduct and PLP-17adduct, as well as key nearby residues.

FIGS. 8A-B. (A) Overlay of in silico model of compound PLP-40 adduct andPLP-17 adduct, (B) Overlay of in silico model of compound PLP-41 adductand PLP-17 adduct.

FIG. 9. Inhibition of the hERG channel by 17 (hERG CHO-K1 cell line,detection method: automated patch-clamp).

FIG. 10. Inhibition of microsomal cytochromes P450 by 17.

FIGS. 11A-B. Time-dependent Loss of (A) imipramine, propranolol,terfenadine, and verapamil and (B) 17 in human liver microsomes (HLM).

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

As part of an ongoing effort to develop antiepileptic drugs, there iscontinued interest in new GABA analogs that inactivate GABA-AT by newmechanisms. Compounds with a leaving group adjacent to the carbanionformed after the γ-proton removal seem to inactivate GABA-AT by anenamine mechanism. To demonstrate certain non-limiting embodiments ofthis invention, a series of conformationally-restricted,tetrahydrothiophene-based analogs (FIG. 3) was synthesized. Suchcompounds have a properly-positioned leaving group that could facilitatea ring-opening mechanism in the inactivation of GABA-AT (Scheme 2). Asdescribed below, the synthesis, biological evaluation, mechanisticstudies, including mass spectral and X-ray crystallographic results ofthese analogs reveal an unexpected inactivation mechanism.

The syntheses of analogs 17-19 are shown in Scheme 3, starting fromcommercially available D-cysteine methyl ester hydrochloride (23). Theroute up to the generation of dihydrothiophene 28 was achieved byfollowing a modified procedure from Adam et al. Reduction of 28 bymagnesium in methanol resulted in diastereomers 29 and 30, which wereseparable by flash column chromatography. Deprotection of the aminogroup and hydrolysis of the ester in 29 and 30 using aqueous HClprovided the desired analogs 17 and 18, respectively. Oxidation of 29 or30 by MnSO₄ and H₂O₂ resulted in a 1:1 mixture of the correspondingsulfones (31) as a result of epimerization at the C-2 position.Subsequent deprotection of the amino group and hydrolysis of the esterin 31 using aqueous HCl gave desired analog 19. Synthesis of compounds20-22 followed an identical route starting from L-cysteine methyl esterhydrochloride. The purity of compounds 17-22 was confirmed by HPLC andHRMS, which showed that there was none of the correspondingdihydrothiophene analog of 17, a known inactivator of GABA-AT.

Preliminary in vitro results showed that 19-22 were weak reversibleinhibitors, while 17 and 18 were potent inactivators of GABA-AT (Table1). The kinetic constants for inactivation of GABA-AT by 17 and 18 couldnot be determined accurately under optimal conditions (pH 8.5, 25° C.),where the enzyme exhibited maximum activity, because inactivationoccurred too rapidly. The kinetic constants for inactivation of GABA-ATby 17 and 18 were instead measured under non-optimal conditions (pH 6.5,25° C.) using a Kitz and Wilson replot. (Kitz, R.; Wilson, I. B. J.Biol. Chem. 1962, 237, 3245-3249.) From k_(inact)/K₁ values (Table 1),it was concluded that 17 is eight times more efficient an inactivator ofGABA-AT than vigabatrin (with an inactivation rate constant almost 20times that of vigabatrin), and 18 is half as efficient as vigabatrin.

TABLE 1 Kinetic constants for the inhibition and inactivation of GABA-ATby 17-19 K_(I) K_(inact) K_(inact)/K_(I) K_(i) Compound (mM) (min⁻¹)(min⁻¹ mM⁻¹) (mM) 17 0.182 0.17 0.93 — 18 2.23 0.12 0.05 — 19 — — — 3.2± 0.7 20 — — — 3.4 ± 0.8 21 — — — 3.3 ± 0.7 22 — — — 7.5 ± 0.7(S)-vigabatrin 3.2 0.37 0.11 —

The X-ray crystal structure GABA-AT inactivated by 17 (at 1.66 Å) showedthat the inactivating metabolite contained a buckled 5-membered ringcovalently bound to PLP, and Lys-329 was not covalently modified. Thefinal ligand interpretation is strongly supported by the electrondensity of the simulated annealing omit map (F_(O)-F_(C), FIG. 4A).Furthermore, the omit map density at a higher contour level alsorevealed that the refined atom positions of the dihydrothiophene ringwere accurate, resulting in a nonplanar ring (FIG. 4B,C).(S)-4-Amino-4,5-dihydro-2-thiophenecarboxylic acid, the correspondingdihydrothiophene analogue of 17, is a known inactivator of GABA-AT thatinactivates by an aromatization mechanism resulting in a thiophene ring;it also is an inactivator of aspartate aminotransferase. Here, thecrystal structure of 17-inactivated GABA-AT suggests that theinactivation of GABA-AT by 17 is likely to follow the mechanism shown inScheme 4. The resulting Schiff base (32) from the reaction of 17 and thelysine-bound PLP on GABA-AT undergoes γ-proton removal, leading toenamine 34. The crystal structure revealed that 34 was stabilized in theactive site by an interaction between its carboxylate group and theguanidinum group of Arg-192, by the interaction between the enaminealkene and the phenyl ring of Phe-189, and the sulfur atom in 34 is inclose proximity (3.3 Å) to a carboxyl oxygen atom of Glu-270 (FIG. 5A).Without limitation to any one theory or mode of operation, this distancesuggests a weak nonbonded interaction between the divalent sulfur andthe carboxyl carbonyl oxygen. Weak nonbonded S . . . O and S . . . Ninteractions have been reported, but are mainly characterized asstabilizing forces of protein structures and of some organic sulfurcompounds. Until now, all reported weak nonbonded interactions have beenintramolecular. It is believed that no intermolecular weak nonbonded S .. . O and S . . . N interactions have been reported. However, thedistance and directionality of intermolecular nonbonded S . . . Ointeractions has been suggested in theoretical studies. Forintermolecular nonbonded S . . . O interactions, the nucleophilic O atomapproaches the S atom from the backside of the S—Y and S—Z bonds (theσ_(s)* direction), and the S atom lies in the direction of the π orbitalof O (the π_(o) direction) (FIG. 5B). The stabilization of this S . . .O═C interaction is described by an π_(o)→σ_(s)* orbital interaction, and3.3 Å is well within the predicted distance. As shown in FIG. 5A, thedirectionality of the interaction between the sulfur atom in metabolite34 and the carboxyl group of Glu-270 matches the description of thedirectionality of theoretical intermolecular nonbonded S . . . Ointeractions. Therefore, the interaction between the sulfur atom inmetabolite 34 and the carboxyl group of Glu-270 might be the firstreported example of an intermolecular nonbonded S . . . O interaction,which contributes to the stabilization of metabolite 34 in the activesite of GABA-AT.

Metabolomics (via ESI-mass spectrometry) was run on a sample of GABA-AT,inactivated by 17, but the presence of 34 was not observed. (See Example18.) When a small amount of formic acid was added to another sample ofGABA-AT inactivated by 17 to disrupt H-bonding before running thespectrum, metabolite 36 was detected instead of 34. Through massspectral analysis, fragmentation data for m/z 144.9954 confirmed thestructure of 36, the likely result of hydrolysis of 34 (Scheme 5).

Treatment of [7-³H]PLP-reconstituted GABA-AT with 17 was performed todetermine the fate of the coenzyme upon inactivation. (See Examples15-17.) A solution of 1 mM PMP and 1 mM PLP was treated identically ascontrols. The results showed that the denaturation of GABA-AT,inactivated by 17, released PMP exclusively (FIG. 6).

Results from the radioactive-labeling experiment and metabolomicssuggested that metabolite 34 was not stable outside of the active siteand would undergo hydrolysis to produce PMP and 36, supporting theproposed mechanism for the inactivation of GABA-AT by 17 shown in Scheme4.

If the interaction between the sulfur atom in 34 and the O═C of Glu-270is an intermolecular nonbonded S . . . O interaction, then thecorresponding cyclopentane analog (39) (Scheme 6) should form a lessstable metabolite in the active site of GABA-AT than 34. Compound 39 wassynthesized from (1S,4R)-2-azabicyclo-[2.2.1]hept-5-en-3-one (37)(Scheme 6) and its activity was investigated. The results showed that 39is not an inactivator but is a good competitive inhibitor of GABA-ATwith a K_(i) of 0.87 mM. A computer model of the energy-minimizedhypothetical adduct of 39 bound to PLP after tautomerization anddeprotonation (i.e., the cyclopentene analogue of 34) docked intoGABA-AT using GOLD gave the pose with the highest fitness score that wasalmost identical to that shown in FIG. 5B (FIG. 7). These inhibition andmodeling results further support the role of the sulfur atom inretaining the product in the active site of GABA-AT, therebyinactivating the enzyme.

Two corresponding tetrahydrofuran and tetrahydropyrrole analogs, 40 and41, respectively, were also synthesized, and their activities wereinvestigated. Depending on the local pH in the active site of GABA-AT,there might be a H-bond between the O atom or NH of the analogs and thecarboxyl group of Glu-270, which might result in a tighter binding ofthe final metabolite to the active site.

Computer modeling for the energy-minimized hypothetical PLP-40 adductand PLP-41 adducts was carried out. These adducts were docked intoGABA-AT (FIGS. 8A and 8B, respectively). The pose with the highestfitness score of the PLP-40 adduct places the oxygen atom in the PLP-40adduct at a farther distance from the oxygen atom in Glu-270 than thecorresponding distance between the sulfur atom in the crystal structureof the PLP-17 adduct and the oxygen atom in Glu-270. This may be thatthe negative charge on the carboxylate of Glu-270 repulses the partialnegative charge on the oxygen atom in the PLP-40 adduct. The pose withthe highest fitness score of the PLP-41 adduct places the nitrogen atomin the PLP-41 adduct at an even farther distance from the oxygen atom inGlu-270 than the corresponding distance between the sulfur atom in thecrystal structure of the PLP-17 adduct and the oxygen atom in Glu-270.With a distance of 4.2 Å, it is unlikely that there is a hydrogen bondinteraction between the nitrogen atom in the PLP-41 adduct and theoxygen atom in Glu-270. In both poses of the PLP-40 adduct and thePLP-41 adduct, the tetrahydrofuran ring and the tetrahydropyrrole ringare smaller than the tetrahydrothiophene ring, which forces the anglesof the carboxylate groups on these rings closer to 109.5°. Since thePLP-40 adduct and the PLP-41 adduct likely favor the interaction withArg-192, they would place the oxygen atom and nitrogen atom at fartherdistances from Glu-270, as the poses suggested.

Compound 40 was synthesized, starting from commercially available4,5-dibromo-2-furoic acid (42) (Scheme 7). Selective debromination of 42using zinc metal, followed by esterification, provided 43. Directcopper-catalyzed amidation of 43 with tert-butyl carbamate, underBuchwald conditions, generated 44. The reduction of 44 by Rh/C resultedin cis-products exclusively, just as reported by Walker et al. (Walker,D.; Wishka, D.; Beagley, P.; Turner, G.; Solesbury, N. Synthesis(Stuttg). 2011, 2011 (07), 1113-1119.) A mixture of enantiomers 45 and46 was hydrolyzed and then coupled with (−)-menthol to provide a mixtureof diastereomers 49 and 50. HPLC using a C18 column, a C8 column, or asemi-prep Whelk-O1 chiral column failed to separate 49 and 50.Subsequent hydrolysis and deprotection of the Boc group in 49 and 50gave a mixture of enantiomers 40 and 51. 1D NOE (not shown) confirmedthat the carboxylic group and the amino group were on same side of thetetrahydrofuran ring. Meanwhile, the Boc groups in enantiomers 45 and 46were deprotected, and the resulting amines 52 and 53 were coupled with(S)-(+)-Mosher acid chloride to afford diastereomeric amides 54 and 55,which were separated by HPLC using a semi-prep Whelk-O1 chiral column.Subsequent 2-step hydrolysis of 54 and 55 with LiOH and HCl 6M afforded40 and 51, respectively.

Compound 41 was synthesized, starting from commercially available 52(Scheme 8). Surprisingly, the removal of the Fmoc group by piperidine inDMF at room temperature or at 50° C. was not successful, and 52 wasrecovered quantitatively. The Fmoc group, however, was successfullyremoved by diethylamine in acetonitrile. The Boc group was then removedby TFA in CH₂Cl₂. In a similar fashion, (2R, 4S) tetrahydropyrroleanalog 54 was synthesized from commercially available 53 (Scheme 8).

Preliminary results showed that the mixture of enantiomers 40 and 51(Scheme 7) was a mixture of competitive inhibitors of GABA-AT with aK_(i)=0.43 mM. Preliminary results also showed that 41 (Scheme 8) was acompetitive inhibitor of GABA-AT with a K_(i)=1.3 mM. These resultsfurther support the role of the sulfur atom in the PLP-17 adduct inretaining the product in the active site of GABA-AT, therebyinactivating the enzyme.

Compound 17 was also tested for the inhibition of hERG and microsomalcytochromes P450 (CYPs). hERG is a potassium ion channel thatcontributes to the electrical activity of the heart, which coordinatesthe heart's beating. This channel is sensitive to drug binding;therefore, when its ability to conduct electrical current across thecell membrane is compromised, it can result in potentially fatal cardiacadverse effects. The results showed that 17 did not inhibit the activityof the hERG channel (FIG. 9). CYPs are major enzymes that are involvedin drug metabolism. They account for ˜75% of all drug metabolism.Microsomal stability is often performed to predict if a drug will beeliminated too rapidly during drug development. The results showed that17 did not inhibit or induce the seven most common CYPs (1A, 2B6, 2C8,2C9, 2C19, 2D6, and 3A) that are involved in ˜95% of the reactions indrug metabolism (FIG. 10).

Compound 17 was also evaluated for its plasma protein binding and itsmetabolic stability in human liver microsomes (HLM). The results showedthat the plasma protein binding of 17 was only 26%, indicating a highpercentage of free drug in plasma. Measurement of the metabolicstability in HLM was accomplished by incubating 17 with the microsomesand monitoring its disappearance with time using LC-MS/MS. Imipramine,propranolol, terfenadine, and verapamil were run in similar condition ascontrols. The results showed that 17 was metabolized less than 20% over60 minutes in HLM (FIG. 11).

The present invention can also, as would be understood by those skilledin the art, be extended to or include methods using or in conjunctionwith a pharmaceutical composition comprising an inhibitor or inactivatorcompound of the sort described herein in a physiologically or otherwisesuitable formulation. In some embodiments, the present inventionincludes one or more such compounds, as outlined above or discussed morefully below, formulated into compositions together with one or morephysiologically tolerable or acceptable diluents, carriers, adjuvants orvehicles that are collectively referred to herein as carriers.Compositions suitable for such contact or administration can comprisephysiologically acceptable sterile aqueous or non-aqueous solutions,dispersions, suspensions or emulsions. The resulting compositions canbe, in conjunction with the various methods described herein, foradministration or contact with a human/animal enzyme expressed orotherwise present therein. Whether or not in conjunction with apharmaceutical composition, “contacting” means that a GABAaminotransferase and one or more inhibitor/inactivator compounds arebrought together for purpose of binding and/or complexing suchcompound(s) to the enzyme. Amounts of one or more such compoundseffective to affect or otherwise inhibit a GABA aminotransferase may bedetermined empirically, and making such determinations is within theskill in the art. Inhibition, inactivation, affecting or otherwisemodulating GABA aminotransferase activity includes both reduction and/ormitigation, as well as elimination of GABA-AT activity and/or glutamateproduction.

It is understood by those skilled in the art that dosage amount willvary with the activity of a particular inhibitor/inactivator compound,disease state, route of administration, duration of treatment and likefactors well-known in the medical and pharmaceutical arts. In general, asuitable dose will be an amount which is the lowest dose effective toproduce a therapeutic or prophylactic effect. If desired, an effectivedose of such a compound, pharmaceutically acceptable salt thereof orrelated composition may be administered in two or more sub-doses,administered separately over an appropriate period of time.

Methods of preparing pharmaceutical formulations or compositions includethe step of bringing an inhibitor/inactivator compound into associationwith a carrier and, optionally, one or more additional adjuvants oringredients. For example, standard pharmaceutical formulation techniquescan be employed, such as those described in Remington's PharmaceuticalSciences, Mac Publishing Company, Easton, Pa.

Regardless of composition or formulation, those skilled in the art willrecognize various avenues for medicament administration, together withcorresponding factors and parameters to be considered in rendering sucha medicament suitable for administration. Accordingly, with respect toone or more non-limiting embodiments, the present invention provides foruse of one or more inhibitor compounds for the manufacture of amedicament for therapeutic use in the treatment or prevention of diseasestates indicated by high GABA-AT activity, low GABA levels, and/orassociated excessive neuronal activity.

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the compounds, compositions and/or methods ofthe present invention, including the preparation of various smallmolecule GABA-AT inactivator compounds, as are available through thesynthetic methodologies described herein. In comparison with the priorart, the present compounds, compositions and methods provide results anddata which are surprising, unexpected and contrary thereto. While theutility of this invention is illustrated through the use of severalcompounds, structural features and moieties thereof, it will beunderstood by those skilled in the art that comparable results areobtainable with various other compounds, structural features andmoieties thereof, as are commensurate with the scope of this invention.

General Procedures

Chemicals were obtained from TCI America, Sigma-Aldrich, Alfa Aesar, andAmerican Radiolabeled Chemicals, and used as received unless specified.All syntheses were conducted under anhydrous conditions in an atmosphereof argon, using flame-dried apparatus and employing standard techniquesin handling air-sensitive materials, unless otherwise noted. Allsolvents were distilled and stored under an argon or nitrogen atmospherebefore use. ¹H NMR and ¹³C NMR spectra were taken on a Bruker AVANCE III500 spectrometer using CDCl₃, MeOD, (CD₃)₂CO, or D₂O as solvents,recorded in δ (ppm) and referenced to CDCl₃ (7.26 ppm for ¹H NMR and77.16 ppm for ¹³C NMR) or MeOD (3.31 ppm for 1H NMR and 49.00 ppm for¹³C NMR) or (CD₃)₂CO (2.05 ppm for 1H NMR and 29.84 ppm for ¹³C NMR) orD₂O (4.79 ppm for ¹H NMR). Nuclear Overhauser Effect (NOE) correlationexperiments were performed using an Agilent DDR₂ 400 MHz spectrometer.High resolution mass spectra (HRMS) were measured with an Agilent 6210LC-TOF (ESI, APCI, APPI) mass spectrometer. The purity of thesynthesized final compounds was determined by HPLC analysis to be >95%.The column used was a Chiralcel OD-H 5 μm, 4.6×250 mm. After thoroughcolumn equilibration, compounds were eluted with a mobile phase of 2%EtOH in hexanes at 0.6 mL/min. Biochemical assays were performed using aBiotek Synergy H1 microplate reader. Prior to their evaluation, initialexperiments were performed to confirm the synthesized analogues do notinhibit the coupling enzymes utilized in the substrate and inhibitionassays. Metabolomics: LC gradient was employed at a flow rate of 200μL/min on an Agilent 1150 LC system (Agilent, Santa Clara, Calif., USA);mass spectrometry was performed on a Q-Exactive mass spectrometer(Thermo Fisher Scientific, Waltham, Mass., USA). Crystallographic datawere collected on beamlines 23ID-B and 23ID-D of GM/CA@APS of theAdvanced Photon Source (APS) using X-rays of 0.99 Å wavelength andRayonix (formerly MAR-USA) 4×4 tiled CCD detector with a 300 mm²sensitive area.

Example 1

(S)-Methyl2-((tert-butoxycarbonyl)amino)-3-((2-methoxy-2-oxoethyl)thio)propanoate(24). To a stirred light suspension of D-cysteine methyl esterhydrochloride (23, 5 g, 29.1 mmol) and Boc₂O (7 mL, 30.6 mmol) inanhydrous CH₂Cl₂ (250 mL) at 0° C. was added Et₃N (15.4 mL, 111 mmol)over a 10 min period. After addition, the cooling bath was removed, andthe reaction solution was stirred at rt overnight. After being cooled to0° C., methyl bromoacetate (3.3 mL, 35 mmol) was added to the reactionsolution and was stirred for 30 min before removal of the cooling bath.Stirring was continued for 2 h at rt, followed by removal of the bulk ofthe solvent under reduced pressure. The resulting crude mixture wasdiluted with ether (60 mL), washed with water (3×30 mL) and brine (5mL), dried (MgSO₄), and concentrated. Chromatography (ethylacetate/hexanes, 3:7) afforded the desired product as a clear oil (6.79g, 74%). ¹H NMR matched literature value. ¹H NMR (500 MHz, CDCl₃) δ 5.40(d, J=8.2 Hz, 1H), 4.55 (m, 1H), 3.75 (s, 3H), 3.73 (s, 3H), 3.26 (q,J=15.2 Hz, 2H), 3.07 (m, 2H), 1.43 (s, 9H). ¹³C NMR (126 MHz, CDCl₃) δ171.50, 170.61, 155.27, 80.35, 53.16, 52.78, 52.66, 35.02, 33.90, 28.38.H RMS (LC-TOF): Calculated for C₁₂H₂₁NO₆S [M+Na]⁺ 330.0982; found330.1001.

Example 2

(3R,4S)-Methyl4-((tert-butoxycarbonyl)amino)-3-methoxy-3-((trimethylsi-lyl)oxy)tetrahydrothiophene-2-carboxylate(26). To a solution of 24 (6.59 g, 21.4 mmol) in anhydrousdi-chloromethane (100 mL) at 0° C. was added Et₃N (3.3 mL, 23.6 mmol)followed by dropwise addition of TMSOTf (4.25 mL, 23.5 mmol) over 20min. The mixture was stirred for 10 min at 0° C. then allowed to warm tort. After being quenched with saturated sodium bicarbonate (50 mL), theorganic layer was separated and washed with saturated NaHCO₃ (2×50 mL),dried (MgSO₄) and concentrated to obtain crude intermediate 25.

In a separate flask, a solution of lithium tetramethylpiperidide wasprepared by the dropwise addition of n-BuLi (14.0 mL, 22.4 mmol; 1.6 Msolution in hexanes) to a solution of 2,2,6,6-tetramethylpiperidide(4.17 mL, 24.5 mmol) in THF (100 mL) at −78° C. After a brief warm-up tort, the solution was cooled to −78° C., and crude intermediate 25 in THF(50 mL) was added dropwise over 30 min. After the addition, the reactionwas stirred at −78° C. for 30 min and at −40° C. for another 30 min. Thereaction was cooled again to −78° C. and quenched with acetic acid (3mL). The reaction mixture was diluted in ether (100 mL), washed withwater (3×100 mL), 0.5 N HCl (3×30 mL), and brine (10 mL), dried (MgSO₄),and concentrated. The crude product was purified by chromatography(ethyl acetate/hexanes, 3:17) to yield the major diastereomer (26) as awhite solid (2.91 g, 36%). ¹H NMR matched literature value. ¹H NMR (500MHz, CDCl₃) δ 6.76 (d, J=9.5 Hz, 1H), 4.22 (ddd, J=9.6, 5.1, 1.8 Hz,1H), 4.06 (s, 1H), 3.70 (s, 3H), 3.31 (s, 3H), 3.23 (dd, J=10.9, 5.1 Hz,1H), 2.80 (dd, J=10.9, 1.9 Hz, 1H), 1.40 (s, 9H), 0.11 (s, 9H). ¹³C NMR(126 MHz, CDCl₃) δ 173.09, 155.37, 110.31, 79.39, 59.44, 52.83, 50.78,49.65, 36.83, 28.50, 0.89. H RMS (LC-TOF): Calculated for C₁₅H₂₉NO₆SSi[M+Na]⁺ 402.1377; found 402.1388.

Example 3

(S)-Methyl4-((tert-butoxycarbonyl)amino)-4,5-dihydrothiophene-2-carboxylate (28).To a stirred solution of 26 (979 mg, 2.58 mmol) in 1 M HF solution (13mL, prepared by diluting 48% aqueous HF in dry methanol) at rt was addedTBAF (2.84 mL, 2.84 mmol; 1 M solution in THF). The reaction was stirredat rt for 2 h before being cooled in an ice/brine bath. Once cooled,NaBH4 (199 mg, 5.16 mmol) was added in small portions while maintaininga reaction temp of 0° C. Following addition, the reaction was stirredfor 1 h at 0° C. before being quenched with acetone (1.3 mL) and allowedto continue stirring at rt. After 1 h, acetic acid (161 μL) was added,followed by removal most of the solvent under reduced pressure. Theresulting crude mixture was diluted with ethyl acetate (30 mL) andwashed with 1:1 saturated brine:water (15 mL), water (2×15 mL), andbrine (3 mL), dried (MgSO₄), and concentrated to yield the crudealcohol, which was used in the next step without purification.

To a stirred solution of the crude alcohol in CH₂Cl₂ (13 mL) at 0° C.was added Et₃N (1.44 mL, 10.3 mmol), followed by mesyl chloride (400 pL,5.17 mmol) dropwise. The reaction was stirred for 30 min at 0° C. andovernight at rt. After removal of most of the solvent under reducedpressure, the resulting crude mixture was diluted with ethyl ether (30mL), washed with water (2×15 mL), 0.5 M HCl (15 mL), and brine (3 mL),dried (MgSO₄), and concentrated. Chromatography (ethyl acetate/hexanes,3:7) afforded the desired product as a white solid (430 mg, 64%). ¹H NMR(500 MHz, CDCl₃) δ 6.51 (d, J=3.3 Hz, 1H), 5.15 (m, 1H), 4.90 (d, J=9.2Hz, 1H), 3.79 (s, 3H), 3.61 (dd, J=12.3, 8.3 Hz, 1H), 3.13 (dd, J=12.3,4.2 Hz, 1H), 1.43 (s, 9H). ¹³C NMR (126 MHz, CDCl₃) δ 162.68, 154.74,138.37, 131.93, 80.35, 58.29, 52.76, 39.40, 28.44. HRMS (LC-TOF):Calculated for C₁₁H₁₇NO₄S [M+Na]⁺ 282.0770; found 282.0773.

Example 4

(2S,4S)- and (2R,4S)-Methyl4-((tert-butoxycarbonyl)amino)tetrahydrothiophene-2-carboxylate (29 and30): Magnesium turnings (484 mg, 19.9 mmol) were added to a mixture of(S)-methyl4-((tert-butoxycarbonyl)amino)-4,5-dihydrothiophene-2-carboxylate (28,430 mg, 1.66 mmol) and NH₄Cl (5.33 g, 99.6 mmol) in MeOH (15 mL), andthe resulting mixture was vigorously stirred overnight at rt. Afterremoval of most of the solvent under reduced pressure, the resultingcrude mixture was diluted with water (20 mL) and extracted with ethylether (3×40 mL). The combined organics were washed with brine (2 mL),dried (MgSO₄), and concentrated. Chromatography (ethyl ether/toluene,2:8) afforded 29 (109 mg, 25%) and 30 (141 mg, 32%) as white solids.(29): ¹H NMR (500 MHz, CDCl₃) δ 5.86 (d, J=7.6 Hz, 1H), 4.55-4.40 (m,1H), 3.97 (dd, J=8.6, 3.1 Hz, 1H), 3.72 (s, 3H), 3.11 (dd, J=11.0, 5.0Hz, 1H), 2.92 (dd, J=11.3, 3.6 Hz, 1H), 2.37-2.26 (m, 1H), 2.17 (ddd,J=14.0, 8.5, 6.0 Hz, 1H), 1.41 (s, 9H); ¹³C NMR (126 MHz, CDCl₃)δ175.10, 155.31, 79.48, 55.54, 52.95, 45.68, 40.16, 37.28, 28.48; HRMS(LC-TOF): Calculated for C₁₁H₁₉NO₄S [M+Na]⁺ 284.0927, found 284.0931.(30): ¹H NMR (500 MHz, (CD₃)₂CO) δ 6.25 (d, J=3.4 Hz, 1H), 4.49 (tt,J=7.4, 3.8 Hz, 1H), 4.06 (dd, J=7.5, 5.1 Hz, 1H), 3.69 (s, 3H), 3.14(dd, J=10.5, 5.6 Hz, 1H), 2.78 (dd, J=10.5, 6.2 Hz, 1H), 2.42 (dt,J=13.0, 5.3 Hz, 1H), 2.15-2.05 (m, 1H), 1.42 (s, 9H); ¹³C NMR (126 MHz,(CD₃)₂CO) δ 174.00, 155.92, 78.96, 55.98, 52.52, 44.49, 37.69, 37.53,28.53; HRMS (LC-TOF): Calculated for C₁₁H₁₉NO₄S [M+Na]⁺ 284.0927, found284.0931.

Example 5

(2S,4S)-4-Aminotetrahydrothiophene-2-carboxylic acid hydrochloride (17):Boc-protected amino acid ester 29 (100 mg, 0.38 mmol) was dissolved in 4N HCl (5 mL) and acetic acid (5 mL). The resulting solution was heatedto 70° C. and stirred for 5 h before being concentrated in vacuo toafford a solid. The solid was purified by ion-exchange chromatography(AG 50W-X8), eluting with a gradient from 0.4 N to 2.0 N HCl, giving thedesired amino acid hydrochloride product as a white solid (63 mg, 89%).¹H NMR (500 MHz, MeOD) δ 4.10 (m, 2H), 3.32 (dd, J=11.8, 5.6 Hz, 1H),3.09 (dd, J=11.8, 5.0 Hz, 1H), 2.52-2.43 (m, 1H). ¹³C NMR (126 MHz,MeOD) δ 177.11, 55.99, 46.14, 37.28, 36.77. HRMS (LC-TOF): Calculatedfor C₅H₉NO₂S[M−H]−146.0281; found 146.0278.

Example 6

(2R,4S)-4-Aminotetrahydrothiophene-2-carboxylic acid hydrochloride (18):Compound 18 (61 mg, 87%) was synthesized from 30 (100 mg, 0.38 mmol)using a similar procedure to that for 17 from 29. ¹H NMR (500 MHz, MeOD)δ 4.15 (p, J=5.9 Hz, 1H), 4.05 (dd, J=7.6, 4.5 Hz, 1H), 3.28 (dd,J=11.6, 5.7 Hz, 1H), 2.95 (dd, J=11.5, 5.7 Hz, 1H), 2.65 (dt, J=13.6,5.0 Hz, 1H), 2.18 (dt, J=13.9, 7.2 Hz, 1H). ¹³C NMR (126 MHz, MeOD) δ175.62, 55.75, 45.19, 37.22, 35.81. HRMS (LC-TOF): Calculated forC₅H₉NO₂S [M−H]-146.0281; found 146.0278.

Example 7

(4S)-Methyl4-((tert-butoxycarbonyl)amino)tetrahydrothiophene-2-carboxylate1,1-dioxide (31): To a stirred solution of 29 (60 mg, 0.23 mmol) andMnSO₄*H₂O (1 mg) in CH₃CN (5 mL) was added at room temperature a mixtureof 30% H₂O₂ (1.15 mmol, 118 μL) and 0.2 M NaHCO₃ (3.4 mL), previouslyprepared at 0° C. After 15 min the reaction was quenched with brine,extracted with ethyl acetate (3×10 mL), dried (Na₂SO₄), andconcentrated. Chromatography (ethyl acetate/hexanes; 1:9) provided thedesired product as a 1:1 mixture of the two diastereomers (55 mg, 81%).¹H NMR (500 MHz, CDCl₃) δ [5.57 (d, J=7.1 Hz); 5.21 (d, J=6.2 Hz), 1H],[4.65 (br s), 4.54 (sex, J=6.6 Hz), 1H], [4.14 (t, J=7.9 Hz), 4.11 (dd,J=9.1, 6.1 Hz), 1H], [3.88 (s), 3.85 (s), 3H], [3.47 (dd, J=13.5, 6.9Hz), 3.42 (dd, J=13.7, 7.0 Hz), 1H], [3.18 (t, J=4.9 Hz), 3.15 (t, J=5.4Hz), 1H], [2.83 (dt, J=13.9, 6.8 Hz), 2.69 (ddd, J=14.1, 8.9, 6.9 Hz),1H], 2.50 (m, 1H), 1.44 (s, 9H). ¹³C NMR (126 MHz, CDCl₃) δ [166.49,165.77], [155.00, 154.78], [80.76, 80.59], [65.14, 64.02], [57.07,56.05], [53.98, 53.78], [45.54, 45.37], [33.30, 32.23], [28.43, 28.41].HRMS (LC-TOF): Calculated for C₁₁H₁₉NO₆S [M+Na]⁺ 316.0825; found316.0833.

Example 8

(4S)-4-Aminotetrahydrothiophene-2-carboxylic acid 1,1-dioxidehydrochloride (19): Compound 19 was synthesized from 31 as aninseparable 1:1 mixture of diastereomers using a procedure similar tothat for 17 from 29 (86%). ¹H NMR (500 MHz, MeOD) δ [4.41 (dd, J=8.6,4.9 Hz), 4.36 (dd, J=9.5, 7.9 Hz), 1H], [4.27 (p, J=7.4 Hz), 4.12 (p,J=8.1 Hz), 1H], [3.73 (dd, J=13.8, 8.1 Hz), 3.68 (dd, J=13.8, 8.0 Hz),1H], [3.35 (dd, J=13.9, 7.1 Hz), 3.29 (dd, J=13.5, 8.1 Hz), 1H], [2.96(ddd, J=14.2, 7.0, 5.0 Hz), 2.84 (dt, J=14.2, 7.2 Hz), 1H], [2.55 (dt,J=13.9, 9.3 Hz), 2.46 (dt, J=14.3, 8.1 Hz), 1H]. ¹³C NMR (126 MHz, MeOD)δ [167.13, 167.04], [66.52, 65.56], [54.61, 54.44], [46.38, 45.40],[31.40, 31.37]. HRMS (LC-TOF): Calculated for C₅H₉NO₄S [M+Na]⁺ 202.0144;found 202.014.

Example 9

(1R,4S)-2-azabicyclo[2.2.1]heptan-3-one (38): Compound 38 was preparedfrom (1S,4R)-2-azabicyclo[2.2.1]hept-5-en-3-one (37) by following apublished procedure (94%). (Evans, C.; McCague, R.; Roberts, S. M.;Sutherland, A. G. J. Chem. Soc., Perkin Trans. 1 1991, 656-657.) ¹H NMRmatched literature value. ¹H NMR (500 MHz, CDCl₃) δ 5.53 (br s, 1H),3.90 (m, 1H), 2.76 (m, 1H), 1.95-1.40 (m, 6H). ¹³C NMR (126 MHz, CDCl₃)δ 181.05, 55.54, 45.04, 41.37, 30.34, 23.79. HRMS (LC-TOF): Calculatedfor C₆H₉NO [M+Na]⁺ 134.0576; found 134.0578.

Example 10

Preparation of (1S,3R)-3-aminocyclopentane-1-carboxylic acid (39):Compound 39 was prepared from 38 by following a published procedure(80%). (Forró, E.; Füllöp, F. Eur. J. Org. Chem. 2008, 2008, 5263-5268.)¹H NMR matched literature value. ¹H NMR (500 MHz, D₂O) δ 3.76 (m, 1H),3.01 (m, 1H), 2.44-1.79 (m, 6H). ¹³C NMR (126 MHz, D₂O) δ 180.01, 51.47,42.40, 33.65, 29.81, 27.68. HRMS (LC-TOF): Calculated for C₆H₁₁NO₂[M+H]⁺ 130.0863; found 130.0864.

Example 11

Preparation of methyl 4-bromofuran-2-carboxylate (43):4,5-Dibromo-2-furoic acid (7.5 g, 27.8 mmol) was suspended in water (83mL) and saturated NH₄OH (27 mL) with vigorous stirring at roomtemperature. Zinc dust (<10 micron, 1.82 g, 27.8 mmol) was added, andthe mixture was stirred at r.t. for 3 h. The reaction mixture wasfiltered through a pad of Celite and then acidified with 2N HCl to pH 2.The filtrate was extracted with ethyl acetate (4×200 mL), combined,dried (Na₂SO₄), and concentrated to afford 3.47 g of white solids. Thiscrude intermediate was dissolved in methanol (90 mL), and concentratedsulfuric acid (0.6 mL) was then added while stirring. The resultingsolution was heated to reflux and stirred overnight. The reactionmixture was then cooled to r.t. and concentrated. Saturated NaHCO₃ (50mL) was added, the resulting suspension was extracted with ethyl ether(4×50 mL). The organic layers were combined, washed with brine (5 mL),dried with MgSO₄, filtered, and concentrated to afford 3.26 g of yellowsolids, which was then recrystallized with hexanes (5 mL) to afford 43as a white solid (2.69 g, 72%). ¹H NMR and ¹³C NMR matched literaturevalues. ¹H NMR (500 MHz, CDCl₃) δ 7.57 (d, J=1.0 Hz, 1H), 7.18 (d, J=1.0Hz, 1H), 3.90 (s, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 158.32, 145.13,144.56, 120.45, 101.41, 52.39. HRMS (LC-ESI): Calculated for C₆H₆BrO₃[M+H]⁺ 204.9495, found 204.9486.

Example 12

Preparation of methyl 4-((tert-butoxycarbonyl)amino)furan-2-carboxylate(44): tert-Butyl carbamate (1.87 g, 15.7 mmol) and a stir bar were addedto an oven-dried sealable vial (10-20 mL). Potassium carbonate (4.53 g,32.8 mmol), CuI (749 mg, 3.93 mmol), and 43 (2.69 g, 13.1 mmol) wereadded to the vial. A septum was put on the vial, and the system was putunder reduced pressure and back-filled with nitrogen twice. Toluene (10mL) and N,N′-dimethylethylenediamine (427 μL, 3.93 mmol) were added viasyringes. The septum was removed, and the vial was quickly sealed. Theresulting mixture was stirred at 110° C. for 21 h. The reaction mixturewas cooled to r.t., filtered through a pad of silica gel, and elutedwith a mixture of CH₂Cl₂ and ethyl acetate (1:1, 80 mL). The organicsolution was then concentrated. Chromatography (ethyl acetate/hexanes;3:7) provided 44 as a white solid (988 mg, 31%). ¹H NMR and ¹³C NMRmatched literature values. ¹H NMR (500 MHz, CDCl₃) δ 7.89 (s, 1H), 7.00(s, 1H), 6.30 (br s, 1H), 3.88 (s, 3H), 1.50 (s, 9H). ¹³C NMR (126 MHz,CDCl₃) δ 159.05, 152.61, 142.87, 134.52, 126.59, 111.35, 81.32, 52.17,28.40. HRMS (LC-ESI): Calculated for C₁₁H₁₅NNaO₅ [M+Na]⁺ 264.0842, found264.0840.

Example 13

Preparation of the mixture of methyl(2S,4S)-4-((tert-butoxycarbonyl)amino)tetrahydrofuran-2-carboxylate (45)and methyl(2R,4R)-4-((tert-butoxycarbonyl)amino)tetrahydrofuran-2-carboxylate(46): Rhodium on carbon (dry basis, 5 wt %, 98.5 mg) was added to a Parrbottle (500 mL). 44 (985 mg, 4.08 mmol) was dissolved in dry methanol(40 mL) and pipetted into the Parr bottle. The Parr bottle was sealedand put under hydrogen at 37 psi. The Parr bottle was shaken at r.t. for48 h. The reaction mixture was filtered through a pad of Celite andwashed with methanol (2×30 mL). The filtrate was concentrated.Chromatography (ethyl acetate/hexanes; 4:6) provided a mixture of 45 and46 as a white solid (845 mg, 84%). ¹H NMR and ¹³C NMR matched literaturevalues. ¹H NMR (500 MHz, CDCl₃) δ 5.15 (br s, 1H), 4.50 (dd, J₁=9.4 Hz,J₂=4.1 Hz, 1H), 4.35 (br s, 1H), 3.99 (dd, J₁=9.4 Hz, J₂=5.3 Hz, 1H),3.89 (br d, J=9.7 Hz, 1H), 3.78 (s, 3H), 2.52 (m, 1H), 2.01 (dt, J₁=13.8Hz, J₂=3.4 Hz, 1H), 1.43 (s, 9H). ¹³C NMR (126 MHz, CDCl₃) δ 173.86,155.40, 79.81, 76.13, 75.13, 52.54, 51.10, 37.17, 28.51. HRMS (LC-ESI):Calculated for C₁₁H₁₉NNaO₅ [M+Na]⁺ 268.1155, found 268.1154.

Example 14

Preparation of the mixture of (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl(2S,4S)-4-((tert-butoxycarbonyl)amino)tetrahydrofuran-2-carboxylate (49)and (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl(2R,4R)-4-((tert-butoxycarbonyl)amino)tetrahydrofuran-2-carboxylate(50): A mixture of 45 and 46 (451 mg, 1.84 mmol) was dissolved inmethanol (7.5 mL), and a solution of LiOH (133 mg, 5.55 mmol, in 2.5 mLwater) was added. The resulting solution was stirred at r.t. overnight.The reaction mixture was concentrated, and the resulting white solid wasdissolved in water (40 mL). The aqueous solution was washed with ethylether (3×20 mL), acidified with 2N HCl to pH 1, and then extracted withethyl acetate (3×40 mL). The ethyl acetate solution was washed withbrine (5 mL), dried with MgSO₄, filtered, and concentrated to afford alight yellow oil (381 mg). This crude oil was dissolved in CH₂Cl₂ (6mL), and (1R, 2S, 5R)-(−)-menthol (258 mg, 1.65 mmol) and4-(dimethylamino)pyridine (20.2 mg, 0.165 mmol) were added. Theresulting solution was cooled to 0° C., and DCC (340 mg, 1.65 mmol) wasadded. The reaction solution was stirred at 0° C. for 5 min and then atr.t. for 4 h. The reaction mixture was filtered through a pad of Celite,and the filtrate was concentrated. The resulting residue was dissolvedin CH₂Cl₂, washed with 0.5N HCl (2×30 mL), saturated NaHCO₃ (30 mL), andbrine (5 mL), and dried with MgSO₄, filtered, and concentrated.Chromatography (ethyl acetate/hexanes; 2:8) provided a mixture of 49 and50 as a clear gel (508 mg, 83%). ¹H NMR and ¹³C NMR showed a mixture of49 and 50 (1:1 ratio). HRMS (LC-ESI): Calculated for C₂₀H₃₅NNaO₅ [M+Na]⁺392.2407, found 392.2409.

Example 15

Preparation of the mixture of(2S,4S)-4-aminotetrahydrofuran-2-carboxylic acid (40) and(2R,4R)-4-aminotetrahydrofuran-2-carboxylic acid (51): A mixture of 49and 50 (490 mg, 1.33 mmol) was dissolved in methanol (7.5 mL), and asolution of LiOH (95.6 mg, 3.99 mmol, in 2.5 mL water) was added. Theresulting solution was stirred at r.t. overnight. The reaction mixturewas concentrated, and the resulting white solid was dissolved in water(30 mL). The aqueous solution was washed with ethyl ether (3×30 mL),acidified with 2N HCl to pH 1, and then extracted with ethyl acetate(3×50 mL). The ethyl acetate solution was washed with brine (5 mL),dried with MgSO₄, filtered, and concentrated to afford a white solid(307 mg). The crude solid was dissolved in dry CH₂Cl₂(7 mL), andtrifluoroacetic acid (2 mL) was added dropwise over 10 min. Theresulting solution was stirred at r.t. for 1 h. The reaction mixture wasconcentrated and purified through HPLC using a C18 column to afford amixture of enantiomers 40 and 51 as a white solid (119 mg, 68%). ¹H NMR(500 MHz, MeOD) δ 4.49 (m, 1H), 4.01 (m, 2H), 3.96 (m, 1H), 2.74 (m,1H), 2.10 (m, 1H). ¹³C NMR (126 MHz, MeOD) δ 175.55, 77.41, 72.56,52.26, 35.34. 1D NOE (not shown) confirmed that the carboxylic group andthe amino group were on same side of the tetrahydrofuran ring.

Example 16

Preparation of methyl(2S,4S)-4-((R)-3,3,3-trifluoro-2-methoxy-2-phenylpropanamido)tetrahydrofuran-2-carboxylate(54) and methyl(2R,4R)-4(R)-3,3,3-trifluoro-2-methoxy-2-phenylpropanamido)tetrahydrofuran-2-carboxylate(55): A mixture of 45 and 46 (82 mg, 0.33 mmol) was dissolved in dryCH₂Cl₂(10 mL), and trifluoroacetic acid (2.5 mL) was added dropwise over10 min. The resulting solution was stirred at r.t. for 1 h. The reactionmixture was concentrated and dried under high vacuum overnight. Thecrude residue was dissolved in dry CH₂Cl₂(10 mL) and cooled to 0° C.Diisopropylethylamine (288 μL, 1.65 mmol) and (S)-(+)-Mosher acidchloride (94 μL, 0.50 mmol) were added. The resulting mixture wasstirred at r.t. for 1 h and then concentrated. Chromatography (ethylacetate/hexanes; 4:6) provided a mixture of 54 and 55 as a white solid(108 mg, 91%). Separation of these two diastereomers was carried out bythe Northwestern University Center for Molecular Innovation and DrugDiscovery using HPLC with a semi-prep Whelk-O1 chiral column. 54 wasobtained as a clear oil (35 mg, 29%), and 55 was obtained as a clear oil(30 mg, 25%). Compound 54: ¹H NMR (500 MHz, CDCl₃) δ 7.67 (d, J=8.0 Hz,1H), 7.50 (m, 2H), 7.41 (m, 3H), 4.71 (m, 1H), 4.57 (dd, J₁=9.7 Hz,J₂=3.2 Hz, 1H), 4.03 (dd, J₁=9.5 Hz, J₂=5.0 Hz, 1H), 3.95 (dt, J₁=9.3Hz, J₂=1.6 Hz, 1H), 3.78 (s, 3H), 3.37 (q, J=1.5 Hz, 3H), 2.58 (dq,J₁=9.7 Hz, J₂=7.0 Hz, 1H), 2.10 (dquint, J₁=13.9 Hz, J₂=1.2 Hz, 1H). ¹³CNMR (126 MHz, CDCl₃) δ 173.79, 166.14, 132.22, 129.68, 128.81, 127.87,125.08, 122.82, 76.18, 74.84, 55.00, 52.60, 49.90, 37.30. HRMS (LC-ESI):Calculated for C₁₆H₁₈F₃NNaO₅ [M+Na]⁺ 384.1029, found 384.1033. Compound55: ¹H NMR (500 MHz, CDCl₃) δ 7.55 (d, J=8.3 Hz, 1H), 7.51 (m, 2H), 7.40(m, 3H), 4.70 (m, 1H), 4.54 (dd, J₁=9.6 Hz, J₂=3.2 Hz, 1H), 4.05 (dd,J₁=9.7 Hz, J₂=4.8 Hz, 1H), 4.01 (dq, J₁=9.5 Hz, J₂=1.2 Hz, 1H), 3.65 (s,3H), 3.44 (q, J=1.6 Hz, 3H), 2.52 (dq, J₁=9.6 Hz, J₂=6.9 Hz, 1H), 1.99(dquint, J₁=13.8 Hz, J₂=1.2 Hz, 1H). ¹³C NMR (126 MHz, CDCl₃) δ 173.73,166.15, 132.73, 129.60, 128.70, 127.62, 124.97, 122.67, 76.15, 75.00,55.13, 52.51, 49.95, 36.99. HRMS (LC-ESI): Calculated for C₁₆H₁₈F₃NNaO₅[M+Na]⁺ 384.1029, found 384.1032.

Example 17

Preparation of (2S,4S)-4-aminotetrahydrofuran-2-carboxylic acid (40): 54(14 mg, 0.039 mmol) was dissolved in methanol (1.5 mL), and a solutionof LiOH (4.7 mg, 0.20 mmol, in 0.5 mL water) was added. The resultingsolution was stirred at r.t. overnight. The reaction mixture wasconcentrated, and the resulting white solid was dissolved in water (4mL). The aqueous solution was washed with ethyl ether (4×4 mL),acidified with 2N HCl to pH 1, and then extracted with ethyl acetate(4×7 mL). The ethyl acetate solution was washed with brine (2 mL), driedwith MgSO₄, filtered, and concentrated to afford a clear oil. The crudeoil was dissolved in 6N HCl (3 mL), and the resulting solution wasstirred at 75° C. for 14 h. The reaction mixture was concentrated andpurified by ion-exchange chromatography (AG 50W-X8), eluting with 2.0 NHCl, and then by HPLC using a C18 column, affording 40 as a white solid(3 mg, 59%).

Example 18

Preparation of (2R,4R)-4-aminotetrahydrofuran-2-carboxylic acid (51): 55(16 mg, 0.044 mmol) was dissolved in methanol (1.5 mL), and a solutionof LiOH (5.3 mg, 0.22 mmol, in 0.5 mL water) was added. The resultingsolution was stirred at r.t. overnight. The reaction mixture wasconcentrated, and the resulting white solid was dissolved in water (4mL). The aqueous solution was washed with ethyl ether (4×4 mL),acidified with 2N HCl to pH 1, and then extracted with ethyl acetate(4×7 mL). The ethyl acetate solution was washed with brine (2 mL), driedwith MgSO₄, filtered, and concentrated to afford a clear oil. The crudeoil was dissolved in 6N HCl (3 mL), and the resulting solution wasstirred at 75° C. for 14 h. The reaction mixture was concentrated andpurified by ion-exchange chromatography (AG 50W-X8), eluting with 2.0 NHCl, and then by HPLC using a C18 column, affording 51 as a white solid(3 mg, 52%). HRMS (LC-APPI): Calculated for C₅H₁₀NO₃ [M+H]⁺ 132.0655,found 132.0658.

Example 19

Preparation of (2S,4S)-4-aminopyrrolidine-2-carboxylic acid (41):(2S,4S)-Boc-4-amino-1-fmoc-pyrrolidine-2-caboxylic acid (52) (232 mg,0.51 mmol) was dissolved in acetonitrile (10 mL), and diethylamine (10mL) was added dropwise. The resulting solution was stirred at r.t. foran hour and then concentrated. The crude solid was dissolved in dryCH₂Cl₂(7 mL), and trifluoroacetic acid (2 mL) was added dropwise over 10min. The resulting solution was stirred at r.t. for 2 h. The reactionmixture was concentrated and purified by ion-exchange chromatography (AG50W-X8), eluting with 2.0 N HCl, and then by HPLC using a C18 column,affording 41 as a white solid (48 mg, 73%). ¹H NMR (500 MHz, D₂O) δ 4.47(t, J=8.8 Hz, 1H), 4.23 (quint, J=7.8 Hz, 1H), 3.88 (dd, J₁=12.9 Hz,J₂=7.9 Hz, 1H), 3.59 (dd, J₁=12.9 Hz, J₂=7.0 Hz, 1H), 2.99 (dt, J₁=14.0Hz, J₂=8.1 Hz, 1H), 2.27 (dt, J₁=13.8 Hz, J₂=8.9 Hz, 1H). ¹³C NMR (126MHz, D₂O) δ 171.24, 59.55, 48.33, 47.43, 32.14. HRMS (LC-ESI):Calculated for C₅H₁₁N₂O₂ [M+H]⁺ 131.0815, found 131.0809.

Example 20

Preparation of (2R,4S)-4-aminopyrrolidine-2-carboxylic acid (54):(2R,4S)-Boc-4-amino-1-Fmoc-pyrrolidine-2-caboxylic acid (53) (250 mg,0.55 mmol) was dissolved in acetonitrile (10 mL), and diethylamine (10mL) was added dropwise. The resulting solution was stirred at r.t. foran hour and then concentrated. The crude solid was dissolved in dryCH₂Cl₂ (7 mL), and trifluoroacetic acid (2 mL) was added dropwise over10 min. The resulting solution was stirred at r.t. for 2 h. The reactionmixture was concentrated and purified by ion-exchange chromatography (AG50W-X8), eluting with 2.0 N HCl, and then by HPLC using a C18 column,affording 54 as a light yellow solid (50 mg, 70%). HRMS (LC-ESI):Calculated for C₅H₁₁N₂O₂ [M+H]⁺ 131.0815, found 131.0816. Calculated forC₅H₁₀N₂NaO₂ [M+Na]⁺ 153.0634, found 153.0636.

Example 21

Purification of GABA Aminotransferase (GABA-AT) from Pig Brain. GABA-ATwas isolated and purified from pig brain by a published procedure. (Koo,Y. K.; Nandi, D.; Silverman, R. B. Arch. Biochem. Biophys. 2000, 374,248-254.) The purified GABA-AT used in these experiments was found tohave a concentration of 6.41 mg/mL with a specific activity of 1.84units/mg.

Example 22

Evaluation of Compounds as Time-Dependent Inhibitors of GABA-AT. GABA-AT(17.5 μL) was incubated in the presence of varying concentrations ofeach compound (70 μL final volume) at 25° C. in 50 mM potassiumpyrophosphate buffer solution, pH 6.5, containing 5 mM α-ketoglutarateand 1 mM (β-mercaptoethanol. Aliquots (10 μL) were withdrawn at timedintervals and were added immediately to the assay solution (137 μL, seebelow) followed by the addition of SSDH (3 μL). The relative enzymeactivity was determined by normalizing the rate of increasing absorbanceat 340 nm to a control. A Kitz and Wilson replot was used to determinethe kinetic constants K₁ and k_(inact).

Example 23

Evaluation of Compounds as Inhibitors of GABA-AT. Inhibition constantswere determined by monitoring GABA-AT activity in the presence of 0-50mM concentrations of synthesized analogues using a coupled assay withthe enzyme succinic semialdehyde dehydrogenase (SSDH). The assaysolution consisted of 10 mM GABA, 5 mM α ketoglutarate, 1 mM NADP⁺, 5 mMβ-mercaptoethanol, and excess SSDH in 50 mM potassium pyrophosphatebuffer, pH 8.5. Enzyme activity was determined by observing the changein absorbance at 340 nm at 25° C. IC₅₀ values were obtained usingnon-linear regression in GraphPad Prism5 software. Subsequent K₁ valueswere determined using the Cheng-Prusoff relationship. (Yung-Chi, C.;Prusoff, W. H. Biochem. Pharmacol. 1973, 22, 3099-3108.)

Example 24

Evaluation of Compounds as Substrates for GABA-AT. Compounds were testedusing an experiment in which the conversion of α-ketoglutarate toL-glutamic acid was monitored as an indication of the rate of PLPreduction to PMP, which in turn corresponds to amine oxidation to thecorresponding aldehyde. Enzyme reactions were prepared at 5 mMconcentrations of compounds in 100 μL pyrophosphate buffer (50 mM, pH8.5) containing 5 mM α-ketoglutarate and 0.13 mg/mL purified GABA-AT andallowed to incubate at room temperature for 24 h. The L-glutamic acidcontent was determined by combining 50 μL of each incubation mixturewith 50 μL of Tris-HCl buffer (100 mM, pH 7.5) containing 100 μMAmpliflu™ Red (Sigma-Aldrich), 0.25 units/mL horseradish peroxidase and0.08 units/mL L-glutamate oxidase in a 96-well black walled plate. Afterincubation at 37° C. for 30 min fluorescence was recorded with the aidof a microplate reader (BioTek Synergy H1) with 530 nm excitation and590 nm emission wavelengths, where fluorescence is proportional to theL-glutamate concentration.

Example 25

Preparation of [7-³H]-Pyridoxal 5′-Phosphate ([7-3H]PLP). To a solutionof pyridoxal 5′-phosphate (PLP) in water (1.8 mL, 0.28 M) was addedthirty drops of 1 M NaOH. The mixture was then cooled to 0° C. in an icebath, and a solution of NaBH₄ (5.86 mg, 0.15 mmol) and NaB[³H]₄ (100mCi) in 450 μL of 0.1 M NaOH was added in small portions and stirred for1 h at 0° C. Concentrated HCl (120 μL) was then added to the solutionvery slowly (the pH of the resulting solution was 4), and the reactionwas stirred for 5 min at 0° C. Ground MnO₂ (200 mg, 2.3 mmol) was added,and the resulting mixture was stirred for 2 h at rt. A solution of 1 MNaOH was added dropwise to bring the pH to 8, and the resulting solutionwas centrifuged. The supernatants were collected and loaded onto a gelfiltration column packed with Bio-Rad AG1-X8 resin (hydroxide form).Water and 5 M acetic acid was used as the mobile phase (gradient from90% water to 0% water, 1.5 mL/min, 300 min). Fractions (10 mL each) werecollected and tested for UV absorption and radioactivity. The fractionswith the desired product were lyophilized and then loaded onto an HPLCwith a Phenomenex Gemini C18 column (4.6 mm×250 mm, 5μ, 110 A). Water(with 0.1% TFA) and acetonitrile (with 0.1% TFA) were used as the mobilephase (5% acetonitrile, 0.5 mL/min, 25 min; then gradient to 95%acetonitrile, 0.5 mL/min, 20 min). Under these conditions, PLP eluted at38 min. Fractions running with the PLP peak were collected, counted forradioactivity using liquid scintillation counting, and then lyophilized,affording [7-³H]PLP.

Example 26

Preparation of [7-³H]PLP-Reconstituted GABA-AT. To potassium phosphatebuffer (0.5 mL, 100 mM, pH 7.4) containing β-mercaptoethanol (0.25 mM)(buffer A) and GABA-AT (170 μg, 1.55 nmol) protected from light wasadded GABA (10 mg, 0.097 mmol). The resulting solution was stirred at rtfor 1 h and then was dialyzed at 4° C. against potassium phosphatebuffer (200 mL, 500 mM, pH 5.5) containing β-mercaptoethanol (0.25 mM)(buffer B) and GABA (4.0 g, 39 mmol) for 3 h. The solution was thendialyzed at 4° C. against 1800 mL of buffer B over-night, followed bydialysis at 4° C. against 4×500 mL of potassium phosphate buffer (100mM, pH 8.0) containing β-mercaptoethanol (0.25 mM) (buffer C) at 1.5 hinterval. An aliquot (1 L) of the dialyzed solution was assayed toconfirm that there was no enzyme activity remaining. To the dialyzedsolution of enzyme (apo-GABA-AT) was added a solution of PLP (40 μL, 20mM) and [³H]PLP (⅕ the amount prepared above) and stirred at rt for 5 huntil the enzyme activity returned and the reactivation was complete.The excess [³H]PLP was removed from the reconstituted GABA-AT solutionby centrifugation with 5×400 μL of buffer C using a 10K molecular weightcutoff filter. The resulting enzyme solution was dialyzed at 4° C.against 2 L of buffer A overnight, affording the [³H]PLP-reconstitutedGABA-AT solution.

Example 27

Inactivation of [7-³H]PLP-Reconstituted GABA-AT by 17 and CofactorAnalysis. A 60 μL portion of buffer A containing [7-³H]PLP-reconstitutedGABA-AT (⅕ the amount prepared above), α-ketoglutarate (3 mM), and 17 (4mM) was protected from light and incubated at rt overnight until theactivity of GABA-AT was less than 1%. To the inactivated enzyme solutionwas added KOH (1 drop, 1 M) to adjust the pH to 11. The mixture wasallowed to stand at rt for 1 h, then trifluoroacetic acid (TFA) (6.7 μL)was added, and it was allowed to stand for another 5 min. The resultingdenatured enzyme solution was centrifuged at 13,400 rpm for 5 min. Thesupernatant was collected, and the pellet was rinsed with 2×50 μL ofbuffer A containing 10% TFA and centrifuged. The supernatant and rinseswere combined and lyophilized. The resulting solid was dissolved in 100μL of a solution containing 1 mM PLP and 1 mM PMP standards and injectedinto an HPLC with an Econosil C18 column (4.6 mm×150 mm, 10μ). Water(with 0.1% TFA) and acetonitrile (with 0.1% TFA) were used as the mobilephase (0% acetonitrile, 0.5 mL/min, 25 min; then gradient to 90%acetonitrile, 1.0 mL/min, 30 min). Under these conditions, PMP eluted at8 min and PLP at 13 min. Fractions were collected every minute, and theradioactivity was measured by liquid scintillation counting.

Example 28

Metabolomics of the Inactitaved GABA-AT. A 50 μL portion of ammoniumbicarbonate buffer (50 mM, pH 7.4) containing GABA-AT (23 μg, 0.21nmol), α-ketoglutarate (5 mM), and 17 (43 mM) was protected from lightand incubated at rt overnight until the activity of GABA-AT was lessthan 1%. Another 50 μL of ammonium bicarbonate buffer (50 mM, pH 7.4)containing GABA-AT (23 μg, 0.21 nmol) and α-ketoglutarate (5 mM) wassubjected to the same condition as a control. After incubation, formicacid (10 μL) was added to each sample, and 20 μL of each resultingsolution was loaded onto a 5-μm Luna C18 column (2 mm i.d.; 150 mm)(Phenomenex, Torrance, Calif., USA). A 30-min LC gradient was employedat a flow rate of 200 μl/min on an Agilent 1150 LC system (Agilent,Santa Clara, Calif., USA). Mass spectrometry was performed on aQ-Exactive mass spectrometer (Thermo Fisher Scientific, Waltham, Mass.,USA). Intact MS spectra were acquired at a resolution of 35,000. Thetop-five most intense ions were selected for fragmentation in adata-dependent acquisition mode. Mass spectra were acquired at aresolution of 17,500.

Example 29

Crystallization and Data Collection. Potassium pyrophosphate buffer (500μL, 50 mM, pH 8.5) containing GABA-AT (200 μg, 1.82 nmol),α-ketoglutarate (5 mM), β-mercaptoethanol (5 mM), and 17 (37 mM) wasprotected from light and incubated at rt overnight until the activity ofGABA-AT was less than 3%. The inactivated GABA-AT was thenbuffer-exchanged into a sodium acetate buffer (40 mM, pH 5.5) bycentrifugation before the initial crystallization screening andoptimization. The crystals were obtained in hanging drops comprising 1μL of 10 mg/mL inactivated GABA-AT and 1 μL reservoir solution,containing 0.1 M ammonium acetate, 0.1 M Bis-Tris (pH 5.5), and 17% w/vPEG 10,000. Diffraction quality crystals grew within 4-5 days at ambienttemperature. For X-ray data collection, these crystals were brieflysoaked in the reservoir solution with additional 20% (v/v) glycerol ascryo-protectant before flash freezing in liquid nitrogen.Crystallographic data were collected on beamlines 23ID-B and 23ID-D ofGM/CA@APS of the Advanced Photon Source (APS) using X-rays of 0.99 Åwavelength and Rayonix (formerly MAR-USA) 4×4 tiled CCD detector with a300 mm² sensitive area. All data were indexed, integrated, and scaledwith HKL2000.

Example 30

Phasing, Model Building, and Refinement. Molecular replacement for theinactivated GABA-AT was carried out using the program Phaser from CCP₄software suite. The tetrameric structure of native GABA-AT from humanbrain was used as starting search model, in which all solvent and PLPmolecules were deleted. Initial R_(free) and R factor of the correctsolution were 29.59% and 28.91%, respectively. The rigid body refinementwas followed by restrained refinement with Refmac5 and further manualmodel inspection and adjustments with Coot. When refinement converged,the Fo-Fc difference maps, before incorporation of ligands in thestructures, show a well-defined electron density for both PLP and theinactivator tetrahydrothiophene (FIG. 4A). The structure of inactivatortetrahydrothiophene was built in Chemdraw (a “Mol” file); the moleculewas regularized and then the chemical restrains were generated inprogram JLigand. The PLP and the inactivator were fitted into theresidual electron density in COOT after the rest of the structure,including most of the solvent molecules, had been refined. The R_(free)and R_(factor) for inactivated GABA-AT were 21.5% and 19.8%,respectively. All structural figures were made in UCSF Chimera.

Example 31

The molecular modeling studies of the PLP adducts with 39, 40, and 41were performed using the GOLD software package, version 5.3 (CambridgeCrystallographic Data Center, Cambridge, UK). The X-ray coordinates ofadduct 34 bound to GABA-AT were used, and the active site was defined asa sphere enclosing residues within 10 Å around 34. The 3D structures of39, 40, and 41 were built using ChemBio Ultra (version 14.0) and wereenergy minimized using an MM2 force field for 1000 iterations and aconvergence value of 0.01 kcal/mol/Å as the termination criterion. Theenergy minimized PLP-39, PLP-40, and PLP-41 complexes were docked in thebinding site of GABA-AT (without 34) and scored using ChemPLP fitnessfunction. All poses generated by the program were visualized; however,the pose with the highest fitness score was used for elucidating thebinding characteristics of 39, 40, and 41 in the GABA-AT active site.Pymol (version 1.1) was used for generating the image in FIG. 7.

Example 32

Inhibition of the hERG Channel. The experiments were performed byEurofins Panlabs (Redmond, Wash. 98052, USA). hERG CHO-K1 cell line wasused. The test concentrations were 0.1 μM, 1 μM, and 10 μM. Theincubation time was 5 min at room temperature, cumulatively. Thedetection method used an automated whole-cell patch clamp. Theexperiments were duplicated, and the % inhibition of the tail currentwas averaged (FIG. 9).

Example 33

Inhibition of Microsomal Cytochromes P450. The experiments wereperformed by Eurofins Panlabs (Redmond, Wash. 98052, USA). CYP1Ainhibition (HLM, phenacetin substrate), CYP2B6 inhibition (HLM,bupropion substrate), CYP2C8 inhibition (HLM, paclitaxel substrate),CYP2C9 inhibition (HLM, diclofenac substrate), CYP2C19 inhibition (HLM,omeprazole substrate), CYP2D6 inhibition (HLM, dextromethorphansubstrate), and CYP3A inhibition (HLM, midazolam and testosteronesubstrates) were tested. The test concentration was 10 μM. Theincubation time was 10 min at 37° C. The detection method was byHPLC-MS/MS. The experiments were duplicated, and the % inhibition of thecontrol values was averaged (FIG. 10).

Example 34

Metabolic Stability in Human Liver Microsomes. The experiments wereperformed by Eurofins Panlabs (Redmond, Wash. 98052, USA). The testconcentration of 17 was 0.1 μM. The incubation time was 0, 15, 30, 45,and 60 min at 37° C. The detection method was by LC-MS/MS. Imipramine,propranolol, terfenadine, and verapamil were run in similar condition ascontrols.

As demonstrated, this invention is directed to a new class of GABA-ATinactivators. Preliminary in vitro results show that 17 is eight timesmore efficient an inactivator of GABA-AT than vigabatrin, anFDA-approved antiepilepsy drug, and 18 is half as efficient asvigabatrin. Mechanistic studies of the inactivation of GABA-AT by 17showed that the sulfur atom in 17 plays a role in keeping the resultingadduct bound to the active site of GABA-AT, thereby inactivating theenzyme. An intermolecular nonbonded interaction between the carboxyloxygen of Glu-270 and the sulfur atom in 17, the first observed exampleof this kind, is believed to be a factor in stabilizing adducts of thissort in the enzyme active site.

We claim:
 1. A method for treating a neurological disease or disorderindicated by low gamma-aminobutyric acid (GABA) levels in a subject inneed thereof, the method comprising administering to the subject acompound of a formula:

or a salt thereof, wherein X is selected from S, SO₂, CH₂, O, and NR,and R is selected from H and straight and branched C1-C6 alkyl moieties,and wherein the neurological disease or disorder is selected from thegroup consisting of epilepsy, Parkinson's disease, Alzheimer's disease,Huntington's chorea, and cocaine addiction.
 2. The method of claim 1,wherein the neurological disorder is epilepsy.
 3. The method of claim 1,wherein in the compound X is S.
 4. The method of claim 3, wherein thecompound is of a formula:


5. The method of claim 4, wherein in the compound the amino substituentand the carboxy substituent have a cis stereochemical relationship. 6.The method of claim 1, wherein the compound is an ammonium salt.
 7. Themethod of claim 6, wherein the counter ion of the ammonium salt is aconjugate base of a protic acid.
 8. A method for treating a neurologicaldisease or disorder indicated by low gamma-aminobutyric acid (GABA)levels in a subject in need thereof, the method comprising administeringto the subject a compound of a formula:

or a salt thereof, wherein the neurological disorder is selected fromthe group consisting of epilepsy, Parkinson's disease, Alzheimer'sdisease, Huntington's chorea, and cocaine addiction.
 9. The method ofclaim 8, wherein the neurological disorder is epilepsy.
 10. The methodof claim 8, wherein the compound is of a formula:


11. The method of claim 10, wherein the compound is a salt comprising anammonio substituent, a carboxylate substituent or a combination thereof.12. The method of claim 11, wherein the compound is an ammonium salt.13. The method of claim 12, wherein the counter ion of the ammonium saltis a conjugate base of a protic acid.
 14. A method for treating epilepsyindicated by low gamma-aminobutyric acid (GABA) levels in a subject inneed thereof, the method comprising administering to the subject acompound of a formula:

or a salt thereof.
 15. The method of claim 14, wherein the compound isof a formula:


16. The method of claim 15, wherein the compound is a salt comprising anammonio substituent, a carboxylate substituent or a combination thereof.17. The method of claim 16, wherein the compound is an ammonium salt.18. The method of claim 17, wherein the counter ion of the ammonium saltis a conjugate base of a protic acid.